A top WHO influenza expert said Monday there was still no evidence H7N9 bird flu was spreading in a "sustained" way between people in China, despite the possibility some family members may have infected one another.
China announced on March 31 that the virus had been discovered in humans for the first time and had confirmed 21 deaths and 104 cases by Monday with another fatality in Shanghai and two new cases in neighbouring Zhejiang province.
Experts fear the prospect of such a virus mutating into a form easily transmissible between humans, which could then have the potential to trigger a pandemic.
"Right now we do not see evidence of sustained human-to-human transmission", Keiji Fukuda, the World Health Organization's assistant director general for health security and environment, said at a news conference in Shanghai.
Fukuda is part of a WHO team visiting China to study whether H7N9 was spreading among humans, which spent the last three days in Shanghai meeting government officials and visiting an agricultural market.
Chinese health officials have acknowledged so-called "family clusters", where members of a single family have become infected, but have so far declined to put it down to human-to-human transmission.
Commenting on the clusters, Fukuda said that based on all the available information, "it's not clear why we have these small clusters".
He said families where more than one person had contracted the virus may have caught it from animals, the environment or one another.
"With other avian influenza viruses we have seen where you can have limited, person-to-person transmission, so there's always the possibility," Fukuda said.
Health experts differentiate between "sustained" human-to-human transmission and limited transmission, in which family members or medical personnel caring for the ill become infected.
The son of a man who was Shanghai's first case of H7N9 was confirmed to have contracted the virus after an initial test ruled it out, Chinese officials said last week.
The Shanghai government also said the husband of a woman confirmed with the virus had become sick with H7N9, but added there was not enough evidence to verify transmission between them.
"Family clusters in general do not change our understanding of the characteristics of the disease," Feng Zijian, an official of China's disease control centre, said last week.
"It is still passed from poultry to people and there is no evidence of human-to-human transmission," he said.
Fukuda said an "unusual" characteristic of H7N9 was a higher number of cases in older people though the reason was unknown.
He said it would take four to six months to develop a vaccine once the decision was made to do so, though preparatory work had already started.
China said earlier this month that it expects to have a vaccine ready in seven months.
Explore further: WHO team probes bird flu in Shanghai